Oscillating fluid-displacing machines operating in parallel



E. s.l L. BEALE Oct. l, 1946.

OSCILLATING FLUID-DISPLACING MACHINES ,OPERATING IN PARALLEL f1Sheets-Sheet l Filed NOV. 17, 1943 r I Il n 11111111/ ,66' 607 ff A 3412352 3e;2

a o T N E V N Oct. 1, 1946.

OSCILLATING FLUID-DISPLACING MACHINES OPERATING IN PARALLEL Filed NOV`17, 1943 4 Sheets-Sheet 2 Pfg. 2.-

Evelqn Shwar* lomsdomhe. (Segu,

E. s. L. BEALE A 2,408,362

Oct. 1, 1946. E. s. g.. BEALE i v i 2,408,362

OSCILLATING' FLUID-DISPLACING MACHINES OPERATIlNG' IN PARALLEL FiledNov. 17, 1945 4 Sheets-Sheet 3 (Smm/ML 1., www

Oct.' l, 1946.

E. s.. l.. BEALE 2,408,362 A OSCILLAT'INGFLUID-DISPLACING-MACHINESOPERATING IN PARALLEL Filed NOV. 17. 1943 y 4 Sheets-Sheet 4 NVENTOR2.2. AT'roRNEY.

` ating machines. c

Patented Oct. l, 1946 OSCILLATING FLUID-DISPLACING MA- CHINES OPERATINGIN PARALLEL Evelyn Stewart Lansdowne Beale, Staines, England, assignorto Alan Muntz & Company Limited, Hounslow, England, a companyY of GreatBritain Application' November 17, 1943, Serial No. 510,638 In GreatBritain November 25, 1942 15 claims. (ci. iso- 97) This inventionrelates to oscillating fluid-displacing machines arranged to operate inparallel on a common mamas either pumps or motors, ,and liable tovariation in the phase-relationship of their respective oscillatingduid-displacing elements.

The invention is concerned especially, but not exclusively, withVinternal-combustion-operated air-compressors and .motive gas generatorsof the free-piston type, in which the oscillating element rincludesapower piston and a compressor piston arranged to reciprocate in unisonyand unconstrained `by any continuously-rotatable crankshaft, so that theperiod of reciprocation depends on vthe inertia of the free-pistonassembly, the

gas pressures acting on it, and the restraint due to friction and thedriving of accessories.

When oscillating fluid-displacing machines are .delivering uid to-ordrawing fluid from a common main iny parallel, the magnitude of thefluid pulsations in the main depends upon the phaserelationship of the4several machines, and it is y usuallyrdesirable to maintain such aphase-relationship that the duid-pulsations inthe main are minimised..An object of this invention is to provide means for automaticallycontrolling both the magnitude and the'sign of the relative phasedisplacement of two such parallel-operating machine'sf A further objectisto provide improved means for automatically controlling thephaserelationship of three or more'such parallel-oper- According to thisinvention a combination of at least two oscillating fluid-displacingVmachines arranged tooperatein parallel on a. common main is providedwith means for maintaining automatically a` predetermined phaserelationship ybetween them',-said means includinga source of fluid underpressurea device actuated by one of said machines which servesas amaster for interrupting the flow of said pressure fluid synchronouslyAonly -once per cycle of saidimaster machine, a deviceV4 actuated by theother of said two machines which 3 acts as a slave, saidslave devicebeing connected to receive the interruptedflow of pressure fluid saidintendedphase relationship. Synchronously herein means during likeportions of successive cycles of the oscillating machine concerned.

According to this invention in a. further aspect,

4a combination of at least three oscillating iluiddisplacing machinesarranged to operate in parallel on a common main is provided with meansfory maintaining automatically a predetermined phase-relationshipbetween them, said means including a source of fluid under pressure, adevice actuated by one of said machines which serves as a master forinterrupting the flow of said pressurefluid synchronously only once percycle of,

said master machine, two devices actuated respectively by the other twoof said machines which serve as slaves, said two slave devices beingconnected to receive in parallel the interrupted flow of pressure fluidpassed by said master device for interrupting the flow of pressure uidsynchronously only orrce per cycle of the respective slave machines, thearrangement being such that there is normally an overlap of thenon-interrupting period ofl said master device with the non-interruptingperiod ofeach of said slave devices, and two controllers for varying theperiods 'of oscillation of therespective slave machines,

said controllers vbeing responsive to the mean rate passed by saidmaster device and serving for inlterrupting the ,flowfof pressure vfluidsynchronously `only once per cycle ofsaid slave machine, thearrangementbeing such that there is normally an overlap of the non-interruptingperiods rofsaid .two devices, and a, Vcontroller for` varying`the'p'eriod of oscillation of said kslave machine, in

response to variation in the mean rate of flowof 'ipressrefilid'passedlby said two devices in series, sich.. a ense as@mais departure' from of flow of pressure fluidy passed by said slavedevices respectively and being so set that one of them increases, whilethe other decreases, the period of oscillation of its associated slaveIna- 'chine in response to an increase in the flow of `pressure fluidpassed by its associated slave device.

.'I'hus, where three oscillating machines are combined to operate inparallel, each of said three devices maybe so arranged that itsnon-interrupting period occurs over one .of the dead points 'of theoscillating elements. Y

Where it is desired that any of said devices shall .be oscillatory andthat the non-interrupting period of said oscillatory device shall occurin vcourse of the displacement of the oscillating ele-V ment betweenits, dead points, the saidrdevice maybe provided .with a disablingelement adapted to maintain it in interrupting condition duringalternate strokes, e. either the instrokes or the outstrokes.

Tli'e 'word iluidg herein referred to as being v fluid underpreSSure orpressure fluid, means a liquid, electricity or a gas. A rv"Where" thepressure uidis a liquid', forexample oil, the interruptingdevices arehydraulic.

, The master valve inay have a plurality of sepa- ,rate prtsarrangedto'feed'different 'slave valvesy i respectively v;,alternativelypthe.master valve may be arranged to feed a plurality ofslave valves through a common valve port, the ducts connecting thiscommon port to the respective slave valves containing nonreturn valves.

llhercthel oscillating machines are` of the :treepiston ityp'e, 'thefloweinterrupting *devices vare conveniently arranged to oscillate withamplitudes dependent on the stroke of the associated free--y pistonmachines, so that these amplitudes var-y in dependence on theoperatingjconditionsfofithe 5 machines; and in this case compensatingmeans may be provided for reducingoreli'minating:the

in the receiver I8. The air content of the cushrupting period to theperiod'of os'illation'consequent on variation in the operatingconditions.

Each of the controllersforveryiigithejperio. of oscillation of the slavemachines i'niayiin'clude a differential fluid pressure-responsivedeviceboth sides of which are connected to saidsurce of fluid pressure.

' JEmloodim'errts 'of Athe invention `as applied to a 'grou'pof'free-piston"gasgenerat'ors 'will Qhe describe'd -loy 'way 'oferzarnple'with lreference lvto the "accompanying -diagrammatic drawings,uin which only those parts "of Lthe 'control 'mechanism withwhich'this'invention'isconcernied =are shown. In "the drawings:

lFig. :'l shows in sectiona "groupo'f `three ga's -geri'eratorsarrangedto loperate inp'ara'llel,

Fig. 2 isa di'agramillustratingtheoperation of Ythisigrrmp,

lFig. '3 shows in section a 'group of '-ve 1gasfgen'erators,

Fig. 4 vis a diagram "illustrating the 'operation 'offtlie'group'shownin Fig.3 Fig's'B anu v6show'nio'clificationsof parts o'f FigLFig.showspart'ovan' arrangement' equivalent tot that shownk in Fig." 3,and ir'igja isaisectron of adetaiiofrig. '1. `In kkth'e "examples hereinV`'to 'be described with referencek to'Figs. 1"t`o"6v thepha'se controlsystem fis hy'dr'auliothe pr'es'sureuid employedloeing 'il. Figs. and "B'show analogous electrica-l '."sy'fstem 'wherein' switches; rsistances'and vv.electromagnetic devices talie fthe plac'es respectively Y of jfthe :piston valves, "restricted princes and fluidpre'ssur-eresponsiv'eadevices of V"the v"hydraulic ,-yst'ems.

In Ting. '1" 'the 'master 'gas-generators denoted *storeenergy"during'theptstrokes of'thepisto'ns'' ions canlbevar-iedby acontrol valve 2I, adapted -fwhenrinitherneutral position shown to seal abalance pipe 22 connecting the two cushions. The Valve 2'I, when -move'dto the left of the neutral position, lputs lthe cushions intocommunication withthe'scavengezair receiver I5 through a nonreturn valve23 which allows air to pass only from ithecushi'onsitofthexreceiver,which will occur dur- 5mg 'that part or" each cycle when the cushionpressure. exceeds the receiver pressure. When the "valve'i is moved tothe right from its neutral position, it connects the cushions to thereceiver i5 "throughea non-return valve-'21H lvv'l'iich allows 'airJtopass` only from the 'receiver l'tolthe'cushions Ywhich Willpccurduring that partei -e'achcycl'e when fthe *cushion pressure Ais Alowerthan the re- .f lceiver vpressure. In Y`such a gas-generator theperiodic time of a cycle can be reduced Lby in- Icreasing the "quantityV'oi --air contained 'in the cushions. Consequently movement ofthe'valve '2l to the 3'left l*and "right lrespe'ctively will'gradually-reduce and vincrease'respectively Vthe cyclical speed of thegas-generator. IThe 7free-'piston -assemoliesp the master'gas-'generator M operate synchronously "a 'hydra'iiiic Uvalve 'showndiagrammatically als'a piston -v'alve the piston f f 4whic'nisV'fonneiion atailrod 225 :offene of th'e free-'piston assemblies. Thisvvalvehas two'similar sets'o'i'portsl and262 operating'in'parallel and*arranged ito remainlopen ltiming v'a part of Aeach Y'cycle when thefree-pistons are approaching, A'passing through and Vreceding from their'innen dead "pintandtoJrem'ain-closed 1during the rest of'thercycieincluding:the outer dead point. 'Theslavegas-:generatorsfs I-and'SZ are' provided `withhydraulic LvalvesVVI and V2 similar to thevai-ve VM, V'except :that each i has only lone set of ports.ZThettwosetsjoirports 261 vand V262 of the "master valvelV-Mcontrollrespectivelythe now of cillin two "pipes :2211 *and 212 l*which branchirom'fa "source "Aof oil `at constant pressure. The vtwo branches "2?'1"and 212' are 'also controlled respec- "tively by't'he twoslavevalves VI'and'VL Each branch includes a metering orifice 313.

'The Lbranch 271 terminates .in a 'cylinder /3I1 j'cont'aining a piston.321v loaded 'by a :spring 331. The piston/B21 "is "connected l"t'o "thecushion "air it "causes'th'e speed ftlie gas-'generator SI to increase,'while the foil 'pressure iin the cylinder f3! 1 u'r'gestlie valvef I'1in "the 'opposite direction. "The 'cylinder 3(1' crnmlmi'cateswi'th anair-colanni chamiermll ."and'with' 'a' leakage V'orifice 351. "The'liranl'i[71?12"te'rrni'n'eites 'in a 'cylinder 312 "containing apiston322 connected "'to' the 'cushion 'air control valve2"I 2 'of the'slave 'gas-'generator 'The pistonvizfis,j loaded b'y'a spring3'321which'fbiasesth'e 'valveiz f2towa'rds 'the position in which v it.causes'thesp'eed 'of the gasfgenera'tor "S2 'to "decrease,while'theilpressure'in the cylinder 552 urges the 'v-alve'ZI"2 inthel'opposite direction. 'The'.cylinde'r 3 f 2 communicates with anair-col- `1rnn "chaniiloe'r L$3112"en'dfwith aleaka'ge 'ori'ce 352."Theirequencyereglating' means 'of the slave gasgenerator 'SZ 'arethusarranged to operate' in the ment of one-third of a cycle as shown, whichcondition vmay beinitially established by manual control, the periodduring 'which theY master valve VM is open overlaps by the equalintervals t1 and tz the periods during vwhich the slave valves VI and V2are open respectively.l The flow capacities of the orifices130, 351 and352 are so selected, in relation to the pressures exerted by the'springs331.and 332 and to the ratio of the intervals trand tz to the periodictime of. the cycle, that the quantity of oil that flows into each of thecylinders 3I1`and 3I2 during onersuch interval is I.suficient to-replacethe quantityrthat escapes' through the leakage orifices 351. and Y'352during onecomplete cycle. The air columns 341 and 342 compensate for the'pulsating admission of oil so that thepistons V321l and 322 remainsubstantially stationary, their' mean positions being asshown. Y e AvvNow fif the 'slave gas-generator S2, which' is leadingthemastergas-generator M by one-third Vofa cycle -in phase, tends toVrun-forl example slow,`v` the interval of overlap .t2 between the nolLinterrupting periods of its valveV V2 and the master valve VM increases,yand the 'resulting infcreased admission of pressure oil to the cylinder3I2 causes the valve 2|2 to befmoved to the right so that thegas-generator S2 .is speeded up. yIf theV slave gas-generator Sl, whichis lagging the masterY gas-generator'by onethi-rd 'of a y 'Cycle invphase, 'tendstorun' for example slow, the valve 'overlap interval: t1decreases, and. Vthe 'resulting decreasedadmission 1 of pressure oil toto thel right sothat the gas-'generator Si is speed- ''Asregulationofthe'power output or delivery pressure of any of the gas-generators maycause -a'variation of the outer and inner dead points of its freepistonsfif ythe oil pressure line control vvalve is operated, as in theexample shown in fFigpl, bya'member moving' proportionally tothe'fdisplacement of the free pistons, the `ratio-iol? 'the valveoverlap interval tothe duration of a cycle -Will -vary with variation oflload on'they gas'- generator. Insome circumstances, and with a carefulchoice of xedopening positions of the 'valve ports, the irregularity yinphase relationship, fdue'tc regulation of thepoweroutput yor delivery`pressure" of the'` gas-generator, can beV kept fwithin tolerable limits.Neverthelessin other circumstances, it may be necessary to provide .thephase-controlling mechanism with means adaptved to` compensate forvvariation in .load on the ga's-generaton In Fig. V1 is shown onepossible compensating arrangement 'which serves to vary theratio ofthe-eiectiveareas of themetering -orice 30 and the'leakage'nrice 351 or352. i, The 'areas `Aof vthe leakage orifices/are icontrolled by needles361 and 362 rigid with pistonsy 311Y `and Y1312 whichwork in cylinders381Land382 and are 4l` ia .sed'bry"springs 391 and 392 insuch adirectinas to increase the leakage; area.` I,Pipes v461 `of five gas-generatorsM, Sl, S2, S3 and S4 sinirthe`cylinder 3l'1 causes the valve 2|.1.tobemoved s and 402 branch from a pipe 40 connected Vto the source '26Aofl oil under constant'pre'ssure and lead to the cylinders 381 and 382so as to admit oil Vwhich urges ythe pistons 311 and 312 in such adirection as to' decrease ythe area of the leakage orifices 351 and 352..The tail rods 251 and 252 are provided with piston valve chambers 60-1and 602 which, while the free pistons of the as# sociated gas-generatorare passing through their outer dead point, open `valves ports 6l1 and6|2 controlling the pipes 401 and v402, the opening period increasing asthe outer dead point increases (i. e. recedes from the middle of thegasgenerator). Meteringorices 621 and 622 arearranged in series with thevalve ports 6|1 and6|2 respectively. Pipes 631 and 632 branch from thepipes 401 and 402 downstream of the valveports and metering orificesjust described and lead directly torestricted leakage orices 641 and 642andalso through metering orices 651 and 652 to valve ports i 661 andV662 forming. additional leakage paths and controlled by the pistonvalvechambers 601 and 602 in such a manner vthatfthe additional leakage pathsare open while the free pistons of the associated gas-generators. arepassing through their inner dead point, the opening perioddecreasing asthe inner dead point increases` (i. e. recedes from the middle of thegasgenerator). y fi The effect on.A the opening period of the slavevalve Vl or V2 of achange in inner dead point is not the same as that ofthe same.l change in outer deadv point, and furthermore the effect of achange in inner dead point is different according to the nature of thechange in the operating conditions of the gas-generator. The leakageorices 641 and 642 are of such a capacity as to provide the bestcompromise for `the inner'dead point.- f K .1

Fig. 3` shows phase-controlling gear Vfor a group ilar tothegas-generators shown in Fig. 1. Like `parts in thes two iigures aredenoted by the same reference numerals, the two plants being gener.-allyvsimilar in arrangement, except for the following differences.

f The master valve VM.r has only one set of ports common to all thebranches to the slave valves,

and each of these branches is provided with a y non-return valve 29permitting oiltov flow only in the direction from the master valvetowards .the slave valve and serving to prevent any ilow ,rofioilbetween the cylinders 3|1, 3l2 etc. during ,intervals-.of overlap of thenon-interrupting pe.-

valve pistonand-the valvebody and which lmask theports during alternatestrokes. These sleeves are drivenby collars 43 and 44 on the valvepistons .arranged to provide the appropriate degree ,of backlash.

In the present example the durationof the Youtstrokeof rthe freeVpistons is about three# guarters vo f that of their irl-stroke, and thefour slavegasgenerators are karranged to lead the `and slave piston 'Ymaster gas-generator respectively by phase displacements which increaseby equal increments from gas-generator S4 `to gas-generator SI, as shownin the upper part of Fig 4 .whichfshows on a base of time, thedisplacements ofthe free pistons. The valves VM', Vl' and V4' arearranged to be masked on the out-strokes, while the valves V2 and V3 aremasked on the instrokes, The lower part ofV Fig. 4 shows' by the shadedrectangles the times during which the valves are closed, the dottedhorizontal lines denoting the times during which the valve ports aremasked by the disabling sleeves. The noninterrupting periods of theindividual valves are shown in the upper part of Fig. 4 where OMdenotesthis period for each of the valves VM', VI' andy V4', O2 theperiod for the valve V2', and O3 that for the valve V3.

- When the gas-generators are running fat a constant speed with theirintended phase relationship, the periods of overlap of opening of themaster valve VM' and of the slave valves Vl', V2', V3' and Vd' are those.denoted by t1, t2, ta and t4 in Fig. 4. Lagging of the slavegas-generators Sl and S3 will cause the periods t1 and t3 to decreaseand consequently the cylinders 3l1 and 3 I3 are so arranged that areduction in the quantity of oil supplied to them through the pistonvalves causes the pistons 32l and 323 to operate the cushion controlvalves 2l1 and 213' in such a sensexas to speedup the gas-generators SIand S3. If, however, the. gas-generators S2 and S4 lag, the periods t2and .te increase, and the cylinders 3JI2`and 314 are setto operateoppositely to the cylinders 3l1 and 3l3, so that an increase in the.quantity of o-il supplied to the cylinders 312 and' 3|4 causes thegas-generators S2l and S4 to be speeded up.

It will thus be apparent that such a combination of ve gas-generators isconstituted by two groups of three gas-generators each of which groupsis arranged similarly to that shown "in Fig. 1, and which have however acommon master gas-generator, the non-interrupting periods of the slavedevices of one group diiering from the non-interrupting periods of theyslave devices of the other group in such a manner that the phase`differences of the ve machines are substantially uniform.

When it is desired to maintain a substantially constant phasedisplacement when the power output or the working pressure of a group ofunits is varied, the opening period of the phasing valve may be variedin laccordance with variation in the inner dead point or the outer deadpoint or both of these points `of the free-piston assembly. This resultmay be attained by providing piston.- type phasing "valves with `portedsleeves controlled by spring-loaded pistons working in cylinderssupplied with pressure fluid in the same way Vas the cylinders 381 and382 in Fig. 1. Alternatively lthe ported sleeves of the phasingvalves'may be controlled by dead-point followers, and these sleeves maybeY actuated by a linkage so .arranged that the opening or closing ofthe phasing ports occurs at a constant percentage of the free-pistonstroke. With phasing ports arranged .to vbe open over a dead point, thevalve opening or cut-ofi point maybe displaced through thel agency for afloating lever in accordance with v'the weighted mean of the inner andthe outer dead points. With phasing ports having an opening period lyingbetween the inner and the `outer dead points, vthe opening or thecut-oir points may be similarly displaced lin accordance with Isuitablyweighted means; a simple but approximate compensation for such ports istodisplace each cut-off point independently by inner andk outerdeadfpoint followers respectively, so as to maintain them at fixeddistances from the dead points.

Fig. 5 shows a compensating mechanism applicable to each of the valvesof the plant shown in Fig. 1. The phasing valve, here denoted by V, isprovided with a ported sleeve 15, the position of which determines thebeginning of the non-interrupting period. An inner dead point follower4.6, of the kind described in application Serial No. 492,865, led June30, 1943, has a connecting member 41 the position of which correspondsto the inner dead point of the free pistons. A similar outer dead pointfollower 48v has a connecting member 49 the position of whichcorresponds to the outer'deadv point of the freepistons. One end of afloating lever 55 is pivotally connected to the connecting member 49 andits other end is coupled by a link 52 to the connecting member 4l. Anintermediate point on thev floating lever is coupled by a link 5l to thevalve sleeve 45. The ratio of the .arms of the lever 50 above and belowthe link 5l is so selected that the opening point of the valve isdisplaced in accordance with the weighted mean of the inner and outerdeadv points.

The uuid-pressure-responsive devices that -operate the cushion aircontrol valves may be of the differential type arranged to balance twohydraulic pressures, instead of beingv spring controlled as in theexamples hereinbefore described. Fig. 6 shows a part of a plant similarto that shown in Fig. l, except that such a balanced system is used, themeans for .compensating for variation in load on the gas-generators areomitted, and a master valve VM having lone set of ports common to bothbranches 211V and l212 is used, these branches having non-return Valves28. Corresponding parts. in Figs. 1 and -6 have the samereferencenu-merals.

In Fig. `6 the iiuid-pressure-respensive device that operates thecushion air control valve 21,1 `of the slave gas-.generator Sl vis acylinder |311 containing the piston 321, which lis springl bia-sed toits central positicm.l The vright-hand end :of this cylinder is suppliedwith oil in exactly the same way as the right-hand endv of thecylinder311 in- Fig. yl, through the valves VM and VI.

'The left-hand end of the cylinder!! 3.11 is provided with a restrictedleakage orifice 55 and is connected by a branch pipe 53, which ycontainsa metering orifice :511, to a pipe l 2.6 communicating vwith the sourceof oil under pressure that snpplies the lmaster phasing valve- VM. Whenthe master and slave machines :are .in the correct phase relationship,the force tending vto move the piston 321 to the right -due to the oilpressure inthe left-hand end of the cylinder i3 I'1 balances the meanforce act-ingl on the right hand face of this piston due to the pressureof .the oil passed by the phasing valves VM and Vl.

This balanced system ,has the advantage that it is unaffected by.accidental variations in the pressure of the .oil supplied to the pipe26.

In ,order to avoid irregularity operation of such hydraulicphase-control mechanisms -due to variation in viscosity' of the Pressureliquid. Awhich is conveniently lubricating oil, the Vsaid meteringorices and leakage ori-ces should -be so designed that the resistancewhich they im- --pose'ito the flow of liquid is substantially pro.-

portional tothe viscosity of the liquid,fthat to say, they shouldprovide a streamline ow with only small kinetic energy eiects. rrrI'husthe metering orices and the leakageVv oriiices will besimilarly affectedby changes in viscosity. A large battery of gas-generators may beconveniently and simply phasedv in two or more groups of two orfourslave gas-generators. all the groups having a common mastergas-generator. In special cases, it -is'possible to control, inaccordance ywith this invention. the phaserelatio'nships of the mastergenerators of two or more separate phase-controlled batteries.

' As -hereinbefore mentioned, an electrical systeml may be used inplaceof a hydraulic vsystem,v4 and the analogy between the. two will" beillustrated by reference to Figs. '1 and 8which are comparable-with Fig:3. The pipe 26- of Fig. 3 isreplaced by a lead226 to one terminalV of acurrent source, 226A, the other terminal of which is grounded. Thehydraulic -valves VM' and VI- to V4' are replaced respectivelybyelectrical switches SwM, Swl, etc. operatively connected with vthe freevpistons of the gas'- generators. Where the gas-generators are, asshown, ofthe usualv opposed-piston type, each switch is convenientlyformed by a ring 80 of insulating material fast on the oscillatory shaft8| of the vpiston 'synchronising mechanism, each ring havingv aconducting segment 82 co-operating witha pair of brushes 83. Thesynchronising mechanism shown in Fig. '1 is of the knownrack-and-pinion'type, so arranged that the maximum amplitude ofoscillation of the shaft 8| is substantially less than 360-. I

*The* sleeves 42 vand 421 to 424 of Fig.'3 are replaced by switches 242,2.421 etc., which are generally similar to the rotary switches SwM,etc., but in which the ring 88A is drivably connected to the shaft 8|-by friction disks 84 (Fig. 8) loaded by acompressionv spring 85, andhave a range of displacement, which is limited by an arm 86 co-operatingwith stops 8.1, between the on and o positions of the segment 88. Thebranch pipes .211 to 214 of Fig. 3Vare relplaced by branch leads 2211`and 2214 respectively, each including a rectifier 229 and a resistor123i), replacing the non-return valves 29 and the orices 30 of Fig, ,3;The cylinder and piston devices ofy Fig. 3 actuating the air-transfervalves 211 to 2|4 are replaced by spring-biased electromagnetic devicesenergized from the appropriate branch lead 2211, 2212, etc., throughztheappropriate pair of switches 2421 and Swl, etc.. vSuch a'device in itssimplest form would be a solenoid having a spring-loadedv armatureadapted to take up a position dependentV on the mean .valueofl theunidirectional current passing through its exciting winding. `Inpractice it isy morecon'venient to use torque motors, one ofwhichappears in Fig. '7. The motor is of the direct-current type and itsarmature 2321 isy connected between ground and the respective branch1ead2111. Its :deld winding 89 is separately energised through aconductor 90 leading directly to the ungrounded sideof the currentvsource 225A. A resistor n9i may be shunted across the armature in orderto reduce sparking at thev contacts of the rotary switches. The armature2321 is coupled by a suitable transmission, indicated diagrammaticallyat 92, to the transfer valve 2I1'and the biasing spring 2331.` Inpractice the Y'efiiciency of the' electromagnetic devices will usuallybe so l'ow that, when they operateas generators, their output will benegligible; thus the rectifiers`229 can be omitted. A

Iclaim: i i ,Y

1. 4A icombination of at least Atwo oscillatory fluid-displacing.machines arranged to operate in parallel on a common main and providedwith 51 means fori maintaining automatically a predetermined phaserelationship between 'them, said meansincluding a source of iiuid underpressure, a device actuated byone of said machines which serves as aVmaster for interrupting the iiow of said pressure iiuid synchronouslyonly once per cycle of said master machine, a device actuated by theother of said two machines which acts as a slave, said slave devicebeing connected to receive the interrupted flow of pressure fluid`passed by said master ,device and serving for interrupting the: flowof'pressure fluid synchronously only once per cycle of said slavemachine-1 the 'arrangement being such that, when said predeterminedphase relationship exists, -there is an overlap between thenon-interrupting periods of said two devices, and a controller forvaryingthe period' of` oscillation of said ,sla-ve machine, inresponsetof variation in the meanrate of flow of pressureuidpassed byIsaid two vdevices in series,.in such a sense as toeliminate departurefrom-said predetermined phase relationship; e

2. A- combination of at least three oscillatory uid-displacing-machinesarranged to operate parallel on a common jmain Iand providedwith means.for'maintaining automatically a predeter-` mined phase` relationshipbetween them, said means rincluding: av source ci uid under pres-A sure,al device actuatedgby oneof said machines which serves as amaster forinterruptingthe ow of saidpressure fluidsynchronously only once per`cycle ofsaid master ,mahineftwo devices, actuatedfrespectivelybytheother two of said machines which act asfslaves, saidftwo slave devicesbeing connected tore ceive in vparallel the interrupted ow ofpressurefluid passed by said master, devicev for interrupting. the flowr ofpressure fluid synchronously Ionly-,once yper cycle of the respectiveslave machines, the arrangement beingv such that, when saidpredetermined phase rela- 45'tionship exists, there isan overlap ofthenoninterrupting periods of-said vvmasterfdevice Ywith thenon-interrupting periods of each of said slave devices, and twocontrollers for varying vtheperiods of oscillationof the respectiveslave ma'- chines, said controllers being responsive to variation in themean rate ofjfl'iow'of pressure fluid passed `by said slave-devicesrespectively and beine.rv so set-that onefof thernjincreaseswhile. theYother decreases, fthe period of oscillatiogof its associated slavevmachines-Kin`- response .to an. rincrease inthe-flow v,of pressure fluidpassed-,by itsassoci'ated slavedevice.;r f

i3. A combination of:l at lleast ,twooscillatoryiiuid-displacingmachines arranged toV operatein parallel onav common-m'ain'and providedwith meansfor maintaining automatically .apredetermined phase relationship {between them, said means including asource of iluid under pressure, a devicefactuated .byeone of saidmachines which servesA as. a *master for interrupting the iiow of s aidpressure fluid synch ronouslyV with the operationof said master machine,[a device actuated by the other of said two'machines which acts as aslave, said slave device being Vconnected to re- O ceivethe'inter'rupted flow of pressure uid passed by Vsaid masterdevice' andserving for interrupt:

ing the iiow of pressure'fiuid synchronously with ther operation 'ofsaid slave machine,v the noninterruptingiperiod of. each-of -saidvdevices occur- 75 ringl 'over onlyoneof the deadpointsl of theassociated oscillatory machine, and the arrangement heingfrsuch that,Whensaidipredetermineii phase relationship exists; therel isa: overi'apzliettizeenil tl-'ienon sinterruptirrgf. periods oi?A said-itwo' devices;and? au controller: for: t'liebperioch of csail-.+- lati'on .ofisaidslave: machine;v in@.respmiseL-toivar-1 ation. irr-.the' meanrateofiiiow. ofpressureffluid; passed;l by said two devices; in':serie-ss. irr. such'i a'. sensei as' tofV eliminate departure: from:said: preedeterminedrphasezrelationslp:.

4i A= combinationzofa aztleast threesoscillators? uiddisplacingamachines `:a1-ranged'. toi operatim paralleli orma common: main andprovided; witin meansi forl maintaining' automatically a.: prede@termined phase relationship f: between. them; said meanssincludingoa'sourceeof Huid-funden. pressure; ae device actuated-hy. onesof said'vmachines, Wlrlielr` serves as a1 master:- frfinterrupting tires now" offsaid?" pressure fluid synchronouslyfwitlr. then-openl ation` off'saidmaster`^ machine; .tivo-devices; actua ated respectively'-v b'y tlielotherttvo-A oi:q said mai-1, chinesswhi'ch act`` as slaves;Vsaidtwo'flslaveidvices being connectedtoreceive' ina paraileltlrentersruptedi-fiw 1or"pressureeililiiipassed?bwsaiiirxrasa ter" device fr'-iriterruptingv-tlrei flow: off pressure fluidesynelironeusiywith theoperation of-thei-resspective-'e slave maeliihes;l theUnone-interrupting period'-ofi'eanniofisaiddevieessocourrirrgfoveronly:OnemEStiie dead pcintsf-of the= associated oscilla toryr machine',- and'the.a arrangement? beings such. that;when\said#predeterminedpliaserrelationship exists,there#issanoverlapiofl'the noir-interruptingiV peribdssofisaid masterdevioewitirethe'rrondnters rupt-iiigfperiod's' 'offeaoii* voffsaidYslavev devices; andL tivo. controllers;forx varying? tiie-period of?asciiT lation off'tiieerespectlveeslavermachines.saidconetrollersflieingresponsive t'ofvvariatiorrdname-mean rateof" pressurefffiidl passedkyfsaidsi'avee devices"resp'eetivelfv andi being1 so .-se't tliaeone-fof? tliem increases? wliil'ei the-f other decreases;` the?. period" ofoscillationj ofi its4 associate'dl sla-vermaachii1ein response t"o aneincrease@ theY iiow.1 off pressureiiiid passedbyj its-f associated save:device:

5 A; combinati'orr--v off at? least; two oscillatoryf iiiii'ddisplaciiigsmachiires' arrangedtovoperateeim parallif orr a'l commonmmaine andi provided with means4` forv maintaining automatiizallyfaprede@ termiiied" phase` relationship Betweem tI'iem'-, said a* deviceactuated ldyfone or`saii-maciiines@whichV servess as a'J masten for"Vinterrupting`r`v the@ domi said? pressure fluid syncl'ironousii'I onlyonce?Y per` cyclF4 or* saidA master lm'aoliine'; a:i device actuated*two-devioesfirr-seriesgsucia asensetas. toselmi lik' 1a nate departure;trom:v said predetermined phase relationships. y

6.=.A. combination; of: at least; live. oscillatom uid-displaeing.machines arranged YIto: operaterin! parallelf onf a. common mainz and'.provided; with mea-ns ior maintaining automatically'a predeten-A minedvphase. relationshipf, between; thenne said; meanssincluding-i a.-sourceof. iluid.u-nderl pressure;l as deviee actuatedby onefof. said.machines' which: serves.- ass. a1- mastern for interrupting@ thef.-ilovev on pressure fluidi synchronously with'. thas operation of saidmaster machines. foiir-h devices-actuated respectively by the otlfiex.four oi-.saiclimachines whichL aot-as-sslavesi ,said ,ioundeyioesibeing; cone nected.- to.: receive.; dansz of; pressere.:v uidiinte-ry rnpted by f said: masten devine;l and. servings to; interruptsaid: flows f of: pressures fluidi ssnnehrofh nonslyl withetheioperation ofi. therespeotiveifslave: machinessadi fives devices: beingioscillators/5- with; their nonf-interriuptingsperiods occurring incourse.- ofi thef4 motion; of theA associ-ated oscillatory man chine-C-betweents dead' points, iives disabling.:v cles. ments. associatedewithsaid rive devicesi respeotively; for; eliminating?. noneinterrupting.periods; one alternate .strokes-thefarrangement: beingesuchf that?,when.saidpredeterminephase:relationship exists, thererisnn overlapcinnamon-interrupting; periodssoi .saidmasten deviceiw-ith lthamour-inten rnpti-ng periodss of each of.r saidrslavedevioes; and,foun'eontrollersfior varying?l the periodse oie: csail-.f-A lation ofA`the; respective;4 slave; machinesl im re.`v` spense to a variation-rthe, mean ratei ori. flora ofq pressurer fluid passedlbistheJrespective..- slaveedef. vices, a iirst pair of saichoontrollersbeingrsofsete f that theyw increase therperfiods of' oscillatiomof the.associatedv rstepair.r of' said slave, machines. inrresponseto. increaseinlthe flowsmf pressure; fluid. passedrbyA` ther associated first-pairofi. saisi slave. devices-,..the;second, pair of.. said contm'llersJbeing soset; that., they. decrease the.. periodsf on oscillation of; theassociated-.seconds pair oi, said. slave machines,x inl response. toiincrease in the; flows` ofi pressure. fluid. passed-by theassociatedisecondpair. ofisaidislave devic.es,.and;.the overlap of.. the.norr-inter-ruptine` per-lodi of.' one: dei/ice. of.. each.oi'sai'dpairswithithatxoi saidlmasterdevioe. occurring during.lkestrokes, oi. the` master. and slave,.whi1e the overlapof. thenon-interrupting period of; the. other deviceof each oi! said pairsJwith. thatl of.' said'. master; device.. occurs` di1rin. ;-.loppositestrolies oil the master.; andlslave...

7.1.5, combinationi of.V at least three oscillatoryIluddisplacingD-machines arranged.' to operaterin, parallel; on acommon, main and; provided; with means .fnrn maintaining. automatically.appredeten minedphase. relationship, hetweenthem, said; meansincludingasource ,olitiilid'unden pressure,-. a hydraulic. valveactuated'. liy.- one of; said; ma.- chineswhichfserves as ama'ster.andlliavi'ng; two.- port's. fori` deliveringf, two separate flows. of."saidl liquid; each4 interrupted-i synchronouslyI once perl cycle of.said master machine, two.- hydraul'i"k valvesactuatedirespectivelr, bythe other. twofoi.' saidmacliir1eswhichlactslslaafes, sailtwoslavevalveslbeing conneotedltoleceiyethe interrupted; ilowssof.' liquid.deliyeredlliy said. two respe.o.. tively, ofthe mastenvalve andservinggtointerruptl the. flow., of. liquid; synchronously; onlyy once;per. cycle of; the. respective, slave machined. the are. rangement;being solch*I that,L whent said;prede-terimiriedlplfiase;reiationshipexistsi therefissan overlapzofimanon-interrupting@eriodsoiisaid.masten valve with thee.nonfinterruptina, periods: o each; oiisaid slava-valves, l.and twolcontrollerseforrvarszr,

. ing the periods of oscillation of the slave machines, said controllersbeing responsive fluid-displacing machines arranged to oprei'aftehin4`parallel on a commonmainaandiprovidedywith means vfmgmairitairiligautomatically a vpredeter- 1 x'nireiif'ph'ase"relationship between them, said 'means including a source of liquidunder pressure,

a hydraulic valve actuated by one of said machines which serves asamaster and having a port for delivering a ilow of said liquidinterrupted synchronously once per cycle of said master machine, twohydraulic valves actuatedrespectively by the other two of said machines,which act as slaves, said two valves being connected to receive inparallel the flow of liquid delivered from said port and serving tointerrupt the ow of liquid synchronously only once per cycle of therespective slave machines, the arrangement being such that, when saidpredetermined phase relationship exists, there is an overlap of thenon-interrupting periods of said master valve with the non-interruptingperiods of each of said slave valves, and two controllers for varyingthe periods of oscillation of the respective slave machines, saidcontrollers being responsive to lvariation in the mean rate of flow ofvpressure liquid passed by said slave valves respectively, in such asense as to eliminate departure from said predetermined phaserelationship.

9. A combination of atleast two free-piston fluid-displacing machinesarranged to operate on a common main and provided with means formaintaining automatically a predetermined phase relationship betweenthem, said means including a source of uid under pressure, a uid pathconnected with said source and including two devices arranged in seriesin said path and actuated respectively by said machines for interruptingthe ilow of pressure uid in said path synchronously with the operationof said machines respectively, the arrangement being such that, whensaid predetermined phase relationship exists, there is an overlapbetween the non-interrupting periods of said two devices, a controllerfor varying the period of oscillation of one of said machines, inresponse to variation in the mean rate of iiow of pressure uid alongsaid path, in such a sense as to eliminate departure from saidpredetermined phase relationship, said devices having oscillatoryelements arranged to oscillate with amplitudes varying with variation inthe length of the free-piston strokes of said machines, and meansv forat least reducing automatically the eil'ect of variation of the ratio ofthe noninterrupting period of each of said devices to its period ofoscillation consequent on variation in the free-piston stroke of theassociated machine.

10. A combination of at least two free-piston fluid-displacing machinesarranged to operate on a common main andprovided with means formaintaining automatically a predetermined phase relationship betweenthem, said means including a source of liquid under pressure, a ductconnected with said source and including two hydraulie valves arrangedin series in said duct and actuated respectively by said machines forinterrupting the flow of liquid in said duct synchronously with theoperation of said machines respectively, the arrangement being suchthat, when said predetermined phase relationship exists, there is anoverlap between the non-interrupting 14 periods of said two valves, acontroller for varying 'the period/of oscillation of one of said ma`chines which acts as a slave, said controller including a hydraulicfluid-pressure-responsive device connected to be supplied with liquid bysaid duct and through a metering port and to discharge liquidlthrough` aleakage port, said con- "-troller" acting to eliminateY departure fromsaid predetermined phase relationship, and said valve actuated by saidslave machine having an oscillatory'element arranged to-oscillate withan amplitude varying with variation in the length of the free-pistonstrokes of said slave machine, and means responsive to variation of theratio of the non-interrupting period of the said valve actuated by saidslave machine to its period of oscillation for varyingthe ratio of theow capacities of said ports so as to compensate at least in-par't forvariation in said free-piston stroke. 'i

11. A free-piston fluid-displacing machine in-= tended for operation inpredetermined phase relationship in parallel with other like machines ona common main and having a controller for varying the period oioscillation of its freepistons, means for conducting a variable supplyof fluid under pressure for actuatingV said controller, and a devicehaving an oscillatory member connected for movement in synchro-nism withsaid free pistons and serving to interrupt synchronously the flow ofpressure fluid through said conducting means, and means for adjustingthe non-interrupting period of said device automatically in response tovariation in at least one of the dead points of said free pistons so asat least to reduce rthe effect of variation of the length of thefreepiston stroke on the ratio of said non-interrupting period to theperiod of oscillation of said free pistons.

12. A free-piston fluid-displacing machine intended for operation inpredetermined phase relationship in parallel with other like machines ona common main and having a controller for varying the period ofoscillation of its free pistons, said controller including a hydraulicactuating device responsive to variation in mean rate of flow of liquidthereto, a duct for admitting liquid to said actuating device andincluding a valve having an oscillatory member connected for movement insynchronism with said free pistons, and means for adjusting thenon-interrupting period of said valve automatically in response tovariation in at least one of the dead points of said free pistons so asat least to reduce the effect of variation of the length of thefree-piston stroke on the ratio of said non-interrupting period to theperiod of oscillation of said free pistons.

13. A combination of at least two oscillatory fluid-displacing machinesarranged to operate in parallel on a common main and provided with meansfor maintaining automatically a predetermined phase relationship betweenthem, said means including a source of fluid under pressure, a deviceactuated by one of said machines which serves as a master forinterrupting the ow of said pressure fluid synchronously only once percycle of said master machine, a device actuated by the other of said twomachines which acts as a slave, said slave device being connected toreceive the interrupted flow of pressure fluid passed by said masterdevice and serving for interrupting the flow of pressure fluidsynchronously only once per cycle of said slave machine, the arrangementbeing such that, when said predetermined phase relationship exists,there is an overlap between the non-interrupting periods of said twodevices,

andacontroiler-fer varyingy thefper-icd ci. oscilla.m tiem cf';saidzslave machine;X inrespnnse tofva-riaf tion ine ti'iegmeanA rate. of.flow oi pressure iiuid. passedf; by said. two: devices: in. series; insuch a. sensefas zt'o: eliminate. departure from .said-prede terminedphaseJ relationship;. Said.; controller infcludingg. a, differentialfluidepressureeresponsive. actuating* device. both sidess. of Whichf.are.` cnnnected'fto.saidsourceiiuid pressure;

14..- ,A1, combination of. at;` least 1 two -free-pistonfluid-displacing, machines arranged to:` oper-ate: on arcommonmainandprovided withmeanssfor, maintaining'. autnmaticallyM apredeterminedphasereiationship betweenthemg. saidfmeans i in.u cluding. a g source;of liquid 1 under. pressure,- a duct. cennectediwithisaidesourceandincludingtwohydrauliczvalyes arranged in serie-sv in-sadzdlletzand.actuated. respectivelyf by saidA machines; for inim terrupting theflow-f of, liquid. in said: ductesyfnl chronously-.- with`v theoperationl of said. machines respectively,Yc the.arraiigaement.:beingsuclr. that,` when. said: predetermined phase.relationshipfeie ists. there. is: anf overlap: between. thenon-interrupting; periods? of said@ two Valves,- a. controller forYvaryings the.A period of oscillation of-l one; of.Y said machines whichiacts: as a: slavesaidncnn trnller. including. a.. differential.hydraulic. prese sure-responsive :device .onef side cf- Whichisl connectedmeAbeesupplied-WithiliquidJoy; said duct and a. meteringgpcrtJand.- tofdischarge.- liquid 1:6 thmugin a leakage;- port; .Y while;the.: other-` side.; oi. saidzdeviceis connectedtabel-suppliedfwithiliquid from; said, sourcef through a meteringA portinde'- pendently of said.I ductl andfto: discharge liquid. through aleakagepcrt,.said controller being; set to.. eliminate departure fromsaid; predetermined phase -relationship.;,

15. AtV least. two:internal?ccmbustion-foperated; gasegenerators; of.`the free-piston type; connected.- to fdeliver gas tofa-y common-main',in"Y combination With means for.A maintaining; them. in. predeterfmined.- phase relationship` eachnof; said tw0- gas-v generators: havingvaf speedfcontrollen; ai. pressure. fluid; circuit for. actuating'vsaid.: controller: and` ade viceeion interruptingsaidcircuit'.syncl'ironnuslyfy oncez per; cycle 0L the-associated:gassgeneratcr; andga'master devicefcapableof interruptinggeach: of. saidcircuits; synchron311513Zata:predetermined.v frequency, .the arrangementbeingsuchithaa'whem said: gasfgener-ators; arei maintained in saidipre``determined pliasefrelationship, there are overlaps? between the?noneinterruptingperiods: of saich master. device andi. of each. ofsa-idslave'.4 devices; variation oil said overlaps: due to;- departure:oi

f. said\ gas--generators. from: said: predeterminismAphase*relationshipcausing: Vania-tion of. thefznea'ny rates: of. nwfo.fiuidfinf said circuits andspeedk correcting actuation' df; said:contrcll'erss

